Quantum solitons and cluster states with well-defined atom number

超低温下的粒子可以表现出惊人的量子效应，比如隧穿和纠缠。通常，这些量子效应对单个粒子是最强的，但对大型多体系统则会减弱。特别是玻色子的多体系统，即允许同时占据同一状态的粒子，可以用集体波函数很好地描述，具有与经典流体相似的性质。目前正在研究这种经典的、类流体的体系和具有单个粒子的系统之间的交叉，并且正在积极讨论观察量子效应所需的粒子数。该项目的目标是探测和研究这种交叉状态下的超冷原子气体的两种状态——“量子亮孤子”和大“簇态”。这两种状态在5到几百个原子的相似实验参数下都有望存在，但它们从相反的方向接近交叉状态。明亮孤子是一种无色散的波包，在不改变其形状的情况下传播，它们表现出非线性流体的典型性质。在粒子数减少的情况下，明亮孤子有望获得单量子物体的特性，如离散隧穿、不确定性关系和纠缠。另一方面，簇态是少数粒子的松散结合态，类似于分子。随着粒子数量的增加，它们将失去量子特性。该项目的目标是通过实验制备具有定义数量的超冷原子的明亮孤子和簇态，并探测原子数变化时系统的性质。该研究将拓宽我们对少体和多体物理边界的理解，并具有推进技术应用的潜力，例如，基于大而复杂量子态的新量子技术的发展。

Particles at ultra-low temperatures can show surprising quantum effects, such as tunnelling and entanglement. Typically, those quantum effects are strongest for individual particles but decrease for large many-body systems. Especially many-body systems of bosons, i.e. particles which allow for a simultaneous occupation of the same state, are well described by a collective wave function, with similar properties as a classical fluid. The crossover between this classical, fluid-like regime and systems with individual particles is currently studied, and the particle number, required to observe quantum effects, is actively debated. The goal of this project is to detect and study two states of ultracold atomic gases in this crossover regime - "quantum bright solitons"' and large "cluster states". Both states are expected to exist for similar experimental parameters with between five and a few hundred atoms, but they approach the crossover regime from opposite sides.Bright solitons are dispersionless wave-packets that propagate without changing their shape, and they present a typical property of nonlinear fluids. For reduced particle number, bright solitons are expected to acquire properties which are characteristic for a single quantum object, such as discrete tunnelling, uncertainty relationships and entanglement. Cluster states on the other hand are loosely bound states of few particles, similar to molecules. They are expected to lose quantum properties with increasing particle number. The goal of the project is to experimentally prepare bright solitons and cluster states with a well-defined number of ultracold atoms, and to probe the properties of the system as the atom number is changed.The research will broaden our understanding of the boundary between few-body and many-body physics, and it has the potential to advance technical applications, e.g. with the development of new quantum technologies based on large and complex quantum states.